1. Field of the Invention
The present invention relates to a CMP (chemical mechanical polishing) apparatus, and more particularly, to a CMP apparatus having a measuring device for measuring a guide ring.
2. Description of the Related Art
Semiconductor fabrication often uses a combination of chemical and mechanical polishing to thin and planarize a thin film coating on a wafer. Typically, the wafer is placed in a polishing head and makes contact with a rotating polishing pad having slurry applied thereto. Often the polishing head holding the wafer also rotates, making the planarization process more uniform.
FIG. 1 is a cross section of a conventional polishing process. The wafers 120 are held in place laterally by the guide rings 150, 160. To facilitate thin film planarization, uniform pressure is applied mechanically from above to the carriers 130 holding the wafers 120 firmly against the polishing pad 110. To aid in maintaining uniform pressure to the carriers 130, a thin carrier film (also referred to as a backing film) 140 is usually attached to each carrier 130. The polishing table (also referred to as a turn table) 100 and polishing pad 110 are rotated at a set speed, while the carriers 130, carrier films 140 and wafers 120 often rotate at a second set speed. During automated loading and unloading, the wafer is secured to the carrier by vacuum pressure via passages (not shown).
During the CMP process, the wafer continuously hits the guide ring, resulting in a score on the guide ring. Out of tolerance scoring seriously affects the uniformity of the wafer during polishing. Moreover, scoring causes the wafer position to shift during loading and unloading, thereby causing the wafer to slide, drop, or break on the polishing table. Thus, when the guide ring has extensive scoring and no longer meets specifications, the guide ring should be discarded. In FIG. 1, symbol 150 shows the guide ring that has no scoring and symbol 160 shows the guide ring that has scoring.
Conventionally, the lifetime of the guide ring is determined by an experiential value by manually measuring the guide ring. This method, however, is difficult and ineffective in measuring the scoring. Thus, it is difficult to control yield by the conventional measuring method.